Extremely high quality coatings may be obtained through a conventional multi-slide coating apparatus, as disclosed in Mercier et al. U.S. Pat. No. 2,761,419 which teaches coating a moving web by metering individual layers of a coating fluid from a supply, through a trough disposed inside a hopper and then through a narrow distribution slot. The fluid is distributed by the slot as a layer uniformly across a downwardly inclined slide surface. The layer of coating fluid flows by gravity down the slide surface. Where more than one layer is coated, the flowing layer meets with the adjacent underlayers of coating fluid which have been similarly metered and distributed through narrow slots. The combined coating fluid layers then flow down the slide surface bridging the gap between the tip of the slide surface and the web, forming a coating bead. The web is carried by a back up roll and is moved across the bead. The fluid layers impinge upon the moving web which picks up the multilayer coating fluids from the slide surface. The coating roll rotates in a direction such that the support moves upwardly in the area of the coating bead. The impingement point is situated approximately in a plane located on the horizontal diameter with reference to the coating roll. Typically, the slide surface makes an angle of between 10 and 45 degrees to the horizontal. In order to stabilize the coating bead, a low pressure chamber may be placed adjacent to the coating roll below the sliding surface, (as shown in Beguin U.S. Pat. No. 2,681,294).
The aforementioned technique has resulted in coating high-quality thin layers on web supports, and is practiced extensively by the photographic industry to coat radiation sensitive emulsions on a web, e.g., a polyester film support. However, the coating rate, for instance, the velocity of the web support when the aforementioned technology is employed is restricted within a narrow range for a given coating fluid. Outside of this range defects such as longitudinal streaks, fogging and pin-point holes may occur in the coating.
In order to increase coating rates, it has been proposed to provide a buffer zone in which the inclined surface changes direction and may even be inclined in an upward direction. A pool of coating material may also be created at the bottom of the slide so that the coating material spans the coating gap beyond the slide surface after its downward flow rate has been reduced to a minimum as shown in Choinski U.S. Pat. No. 4,283,443. In this case the coating material is accelerated in a downwardly direction by the coating device and then redirected to approach the coated web in a tangential direction. A substantial increase in the permissible coating rate range is not, however, obtained since the coating solution must be accelerated from the low velocity of the fluids in the pool to the web velocity.
Another coating device of the type shown in Galer U.S. Pat. No. 2,933,061 is known in which the coating material is applied onto a downwardly moving supporting web by means of a flexible trailing blade. Kobayashi et al. U.S. Pat. No. 4,241,689 shows a fixed coating head which may be used in place of the flexible blade so that a wedge shaped gap between the head and the support is filled with coating material. A pressure controlling chamber is provided below this gap to stabilize the coating material within the gap. Only one coating layer at a time can be applied by this method. Difficulties are also present when it is attempted to apply very thin layers using the method of coating over a slide surface.
A curtain coating device such as shown in Ridley U.S. Pat. No. 4,019,906, in which the coating material is directed over a slide whose final section is vertical is also known. In this arrangement, a supporting web to be coated runs horizontally below the slide. The distance between the slide tip and the support is traversed by the coating material in free fall. This device is only suitable for high coating speeds. The falling coating material undergoes a 90 degree turn. The coating material forms a curtain as it bridges the gap between the tip of the slide coater and the moving web which is more difficult to stabilize then a bead in a bead coating apparatus. Lateral constriction is also present when this method is employed.
It is desirable to provide a device which will allow a greater operating range of coating speeds and more particularly higher coating speeds then previously possible for a given coating material.